1. Field of the Invention
The present invention relates to a toner to be used in image formation employing electrostatic charge development or a toner jet system in an image forming method such as an electrophotographic, electrostatic recording, or electrostatic printing method. In particular, the present invention relates to a color toner with which an image of high fineness and quality can be expressed even if a fixing means is used in which oil for preventing a high-temperature offset is not used or is somewhat used.
2. Description of the Related Art
In recent years, electrophotographic apparatuses have been requested to be constructed of more simplified components to meet a specification that states the necessary features upon image formation such as size and weight reductions and low power consumption while achieving colorization, high fineness, and high image quality.
Thus, image formation of a full-color image with high quality has been attempted in the art because of an increase in market demand for high fineness and quality of an image in electrophotography. In the case of a full-color electrophotographic image, three or four color toners are superimposed on one another to form a full-color image. However, if the color toners for the respective colors are not similarly developed and transferred, color reproduction may be deteriorated or color drift may occur. Those colors are formed with pigments or dyes, so that these materials will exert large influences on the development and the transfer. Furthermore, in a full-color image, fixing property, color mixing property, and offset resistance are important at the time of fixation, so that a binder resin suitable for these properties is selected. However, the binder resin will also exert large influences on the developing and transferring properties. The influences include those of temperature and humidity on the charge amount of toner. Therefore, there is an urgent need to develop a color toner having a stable charge amount even under various environments.
As a measure for solving such problems, there is a method in which various kinds of external additives are added to toners. In particular, for improving various image characteristics such as resolution, density uniformity, and fogging, the addition of various kinds of fine particles to toners to improve charging and transferring properties of the toners has been widely performed.
For such inorganic fine particles, the following are generally used: (i) inorganic fine particles whose surfaces have been treated with a silicone oil, a silicone varnish, or a silane compound; or (ii) inorganic fine particles including surface-treated titania and an inorganic fine particle whose surface has been treated with aminosilane (see JP 05-19528 A, JP 05-61224 A, JP 05-94037 A, JP 05-119517 A, JP 05-139748 A, JP 06-11886 A, and JP 06-11887 A).
Further, for the inorganic fine particles, (iii) those to which two types of inorganic fine particles are added are preferably used (see JP 04-204751 A, JP 04-280255A, JP 04-345168A, JP 04-345169A, JP 04-348354 A, and JP 05-113688 A).
However, even though each of those proposals allows an improvement in electrophotographic characteristics of toner, a sufficient triboelectric charging amount cannot be obtained as a result of standing under high humidity or for a long period with uniform hydrophobic processing being insufficient. Thus, a decrease in image density or fogging may occur. Alternatively, a frictional charge amount may become excessive under low humidity, causing an irregular image density or fogging. Furthermore, the transferring property of a toner is insufficient because the releasing property of the toner from a photoconductive drum to a transfer member is not sufficient. Thus, a decrease in transfer efficiency or a defect of transferred colorant may occur. In other words, there is no way for solving both of the problems. Furthermore, it is not at all satisfactory particularly when applied to a full-color toner.
In JP 01-31442 B, there is proposed a metal oxide powder with a low-bulk density to be provided as an external additive. In this case, the powder particle has an amino group and a hydrophobic group on its surface, where an OH group thereof is blocked, and a specific surface area thereof is at least 50 m2/g. In addition, the surface of the powder particle is charged positive or uncharged. However, in this case, the charging property of the surface of the metal oxide powder is adjusted with a processing agent, so that the charge amount distribution on the surface of the metal oxide powder at the micro level may broaden or the charge amount distribution on the toner may broaden. Therefore, the method disclosed in the document is not preferable.
In each of JP 11-174721 A and JP 11-174726 A, there is disclosed a toner that contains oxides prepared by high-temperature vapor phase method of a silicon halogenated compound and a halogenated compound of a specific metal. In addition, titanium-containing silica is disclosed as the oxide prepared by high-temperature vapor phase method. The silica is vapor-phase oxidized under a high temperature, so that titanium therein can be of crystalline. In addition, the silica contains a large amount of a halogen component, which is inferred to exert an adverse effect. The content of a titanium compound can be high because the addition of titanium is only for the purpose of adjusting the charge of silica. In addition, there is no sufficient consideration given on the transferring property of toner having excellent low-temperature fixing property and oil-less fixing property, which are problems required to be improved.
Furthermore, in JP 2002-029730 A, there is proposed a method for controlling the charging property of the surface of silica particles by coating the surfaces of silica particles with a hydroxide or an oxide of titanium, zirconium, tin, or aluminum in an aqueous system, and subjecting the particles to a surface treatment with alkoxysilane in the aqueous system. However, it is difficult to provide the surfaces of silica particles with sufficient reactivity and adhesion even if the surfaces of silica particles are coated with a hydroxide or an oxide of titanium, zirconium, tin, or aluminum in the aqueous system. It is believed that the characteristics of a different metal existing near the surfaces of silica particles may exert a strong influence on the toner even if the surface treatment is completed favorably. In addition, the presence of such a metal significantly changes charging polarity and surface electric resistance of silica particles, exerting adverse effects on the charging property and charge amount distribution of the toner. Thus, such a method is unfavorable.
As described above, at present, there exists no toner that has good charging property, transferring property, fixing property, and durability while being hardly influenced by temperature and humidity, sufficiently controls and restricts the negative charging property of silica particles.